by Peaky » 20 Mar 2005 12:36
Due to previous questions and references to the subject and various requests in a recent thread, I have compiled this beginners guide on the process of silver soldering.
Before we start I have to say that this is a simple and very rewarding process but also is a skilled procedure that if done incorrectly or without any regard to the safety aspects involved, can be dangerous and therefore not advisable to anyone that is not fully confident in what they are doing, I would say it isn’t a process our younger members should be contemplating and isn’t something you should be doing in the house.
I was professionally taught how to braze and these are my own experiences and advice that has served me well for over 15 years in working for the government engineering laboratories not forgetting the countless hours spent on private projects and jobs, a skill I am now finding many uses for in my role as a locksmith whether it be prototyping new pick ideas or repairing commercial ones.
I can not accept responsibility for any accidents or mishaps that may happen as a result of following this guide, you would be advised to research the subject further and perhaps look in to joining a local night class to fully appreciate the process involved if you are still unsure of what to do. I do believe however, that if you follow this guide you will be soon wondering what all the fuss was about and churning out loads of new shiny picks.
You may have noticed the word braze above,
Brazing is the term used for the joining of two or more materials by using a flame, (usually a gas torch) and a filler rod. A filler rod is a length of metal usually found as round wire or flat bar of various sizes. Depending what materials are being joined, the item’s working temperature, strength required, stresses encounted etc determines what material the filler rod is made of. For the reasons we need to braze items, a filler rod with a high silver content (around 55%) is the best to use, hence the term ‘silver soldering’.
During the silver soldering process closely fitting parts are heated and a filler metal is introduced. As the filler metal becomes liquid it gets sucked into the joint, this is known as ‘capillary action’ when the assembly cools the remaining joint is strong and void free.
As a note, a weld is a join in any material and is not process specific, welding is this joining but still not specific enough, brazing is a form of welding and silver soldering is a form of brazing.
Safety
I know this is the boring bit but there are hidden dangers in silver soldering as well as the obvious ones,
You are working with an open flame and hot items, it is very easy to put a hot torch down onto something and soon have a fire, I always have a fire extinguisher to hand, make sure it’s a water one as hot items can relight if put out with co2 and all other versions are too messy, I also have a CLEAN bucket full of cold CLEAN water so any burnt limbs can be plunged straight in. A pair of safety glasses or goggles should be worn, you shouldn’t need tainted glass as the flame isn’t that hot, in the past I have had a piece of solder flick into my eye which resulted in a painful burn, learn from my mistake. Keep the work area clean, don’t have any combustible materials nearby if only because of the fire hazard. Fireproof gloves are a great benefit but you can cope with just tongs or a pair of pliers etc. Keep the area well ventilated or get an extractor, as there is always the possibility of dangerous fumes and gases coming from the filler rod or work.
Tools
A good torch, and gas supply, I use separate items but these all in one plumbers torches are perfect for the job, you can get simple no thrills ones to adjustable angle heads to piezo electric lighter ones, you can use these pencil torches that look like an over sized pen but the gas will not last long, get the best you can afford.
Gas, most torches can be used with either propane or MAPP gas, Mapp gas is a pre mixed gas and will give out a slightly higher heat than normal propane but either is suitable, I think acetylene is too hot due to the torches used.
Tongs or long nose pliers to handle hot work pieces.
Flux, I use Johnson Matthey EasyFlow flux powder and is good up to 800’c.
Silver solder, I use easyflow solder that is in a flat form as I find it easier to control, I get around 55%+- silver content, the higher the silver content the better, I get mine in 1m lengths.
A metal container full of clean water to quench the work when finished.
A wire brush, wire wool or emery cloth for cleaning,
A decent degreaser, cellulose thinners is acceptable or any thing that evaporates and doesn’t leave a residue.
Clean the work
I cannot stress enough how important it is to get the parts to be soldered as clean as possible. You can use various acids and cleaners but for home use a good degreaser and then a good rub down with either wire wool or wet and dry is more than adequate. The cleaner the work the better the joint, the flux will clean the joint that little bit more but you really do need to make the effort to do the best you can before flux is applied.
Flux the joint
All this cleaning is all well and good but your work is still open to contamination even when you are heating the work up, you’ve spent time cleaning your work the last thing you want to do is contaminate the joint with the oxides forming from the hot metal hitting the oxygen in the air, these oxides sit on the surface of the work and will stop the filler metal penetrating the two halves properly producing a weak and pitted joint. The way to stop this is to either solder the parts like they do in industry i.e. in a furnace where oxygen is a lot less concentrated or you can use a substance called flux.
Flux stops oxygen getting to the work by acting as a protective layer, it also serves as a sponge for any oxides that form during the heating process or were left from cleaning.
Flux comes in many forms but the best for our use is the powder type that you mix with water, there are many different types of flux depending on application, the JM easyflow can be used for stainless and brass/steel but it may be best to check with your supplier for what will be best for your job and filler rod material. Use clean water and only mix as much as you need. Use a clean container to mix it in DO NOT USE THE LID OF THE FLUX CONTAINER this puts damp in the flux and reduces its life and effectiveness. Never reuse flux, once it’s dried its waste. Mix the flux with enough water until it makes a paste similar to toothpaste then spread it along the joint to be made, don’t put too much on only use enough to cover the surface. Flux changes its state when heated and you can use the state as a rough guide to how ready the work is for soldering. As a rough guide, at 100’c the water will boil off, at approx 315’c the flux goes puffy, at approx 435’c the flux isn’t puffy anymore and looks milky, at approx 590’c the flux is now clear and runny, the metal underneath is visible and brighter, this is more or less the right temperature for soldering.
Preparation
Preparation of the work prior to soldering is paramount to ensure a good reliable joint on the finished work.
Clean the entire piece to be soldered with either wire wool or emery paper (use it dry). Remove all traces of rust, glue, oxidants etc the area to be soldered must be as free of foreign matter as possible.
Wipe the work down with a good degreaser or solvent paying particular attention the joint area,
Clamp the work together, this is ideally done with tool makers clamps but small ‘g’ cramps can be used, stay away from vice’s and the such like as the heat soak of these items means that you will take a lot of time (and gas) to heat the work up to the right temperature and in the case of the big metal working vises you probably wont get it to temperature at all, at a push mole grips can be used just don’t grab hold of them when hot, basically use something that isn’t going to melt or zap all the heat away and you will be fine.
Mix an adequate amount of flux for the job in hand, for a guide you would probably need around a thimble full of powder if you was soldering something like a 1 inch round brass plate.
Mix the flux with CLEAN water in a CLEAN container (I use a degreased aerosol cap), mix the water into the powder don’t put the powder into the water, add a little powder at a time mixing thoroughly each time until you get a consistency of McDonalds milkshake, if you get it over runny you can add the slightest amount of powder until you get it right.
Using an adequate tool (I use the silver solder itself) spread a light coating around the joint, you need to go around the whole joint in a continuous run, if the flux runs over parts of the work not to be soldered wipe it off with a clean cloth.
You should now have a piece of work clamped together with flux around the joint.
Hearth
As you should be aware when using a torch the flame and heat doesn’t only go on the joint and nowhere else, it flows around the job and hits the area you are working in.
What I suggest you do is a similar set up to mine, make a hearth.
Go to your local builders merchants and purchase some fire bricks, make sure with the store that they will be suitable for using as a brazing hearth, off the top of my head I think if the brick has been furnace fired it will be suitable but you will have to check this fact.
If you are only ever going to do small jobs you will get away with 6 bricks, 2 as a base and 2 90’ to each other on 2 of the sides on top of each other making a corner with walls 2 bricks high and a base of 2 bricks wide, you do not cement them together so after they have cooled the hearth can be broken up and stored out of the way. If you are doing bigger jobs you can scale it up, I have a base of around 18 bricks with walls on 3 sides around 6 bricks high.
Doing the deed
Right you are all set up now and ready to go, concentrate and think what you are doing especially where you are pointing the torch when not soldering.
The aim of the process is to get the work hot enough all around the area of the joint and not get it too hot, if the work isn’t up to heat properly the solder will just blob and fall off, if it is too hot the work can deform and the flux will burn off making the metal oxidize and the solder not adhere.
Place your work in the center of your hearth and put your eye protection on.
Light you torch and gently start heating the entire area to be joined, you will find that if you hold the torch in one place for too long the flux will burn and the rest of the work will still be to cool, if you move the torch over the work to quickly it will not heat the work up to temperature, the same goes for how far away you hold the torch, you will notice 3 different flame colors on your torch, the biggest outside edge of the flame is a dull light blue then there is usually an inner flame which is a brighter blue then there is the inner most flame which is the smallest of the lot, it is this smallest flame that is the hottest and the very tip of that flame is the hottest part of the entire flame.
You need to be heating the work up by using the part of the flame, which is just ahead of that tip, this is around the right temperature and distance away from the work you need to be for soldering.
Slowly move the torch around the work piece heating the work up uniformly until the flux goes clear, don’t be tempted into trying the filler rod until it does go clear,
When the flux does go clear, dip the end of your filler rod into your flux and gently introduce it to the joint to be made, if all is well and the work is the correct temperature the filler rod will melt and get sucked into the joint, if it’s a small item the filler rod should flow through the entire job and the joint is done, you can check its gone all round by just looking around the entire joint if there is an area not covered run the torch over that part, if it still doesn’t fill the join add a little more silver solder, sometimes flux residue can look like a black hole but only experience will teach you when a hole is not a hole.
The work can now be left to cool in the air or can be quenched in water.
Clean up
You are now left with a black and charred looking work piece, don’t worry most of that is the flux residue and oxidants.
There are many ways to remove the flux residues and such like but most are acids that have to be warmed up so I will not advise on this, what I do at home is just give the joint a good wire brush then emery paper or wire wool to clean the rest of the piece, you will notice that the better the job you did when you very first cleaned the work the easier it is to clean now.
I can’t think of anything else I need to put down in this beginners guide but if anyone wants something clearing up or elaborating on feel free to ask.
I acknowledge that I am no expert on this subject and that there are many other people on site who also know and do this art regularly and that my methods and suggestions may be different to theirs, I do however feel that if this guide is read and understood you will have all the basics to go and have a go yourself.
Good luck and let me know how you all get on.[/u]
